1. Field of the Invention
This invention relates to a system for accurately measuring distances between points.
2. Description of Related Art
Typically in the manufacture of larger parts, it is necessary to accurately measure distances between points on a past to determine if the part was manufactured correctly. This is also true in maintaining and repairing parts which have been previously manufactured. Sometimes it is necessary to determine if the part has warped, has been bent or deformed either through normal wear and tear, or by some outside abnormal force.
One such use would be in determining dimensions on a casing of a steam turbine. Typically these castings may be up to 40 feet long. Points on these castings are measured to determine if the casting meets the specifications set forth in the design of the casting or for the purpose of manufacturing replacement parts for the turbine.
One such way to measure distances is by using interferometry. Typically the light beam from a coherent, monochromatic light source (such as a laser) is split into two beams using a beamsplitter. One beam, the reference beam, is caused to fall onto a photosensitive detector. The second or measurement beam, is allowed to travel through space and strike a reflector positioned in space at the first of two measurement points. The orientation of the target reflector is adjusted so that the reflected beam from the target reflector is returned to the surface of the photosensitive detector. Here the reference and reflected measurement beams interfere. The target reflector is then moved to the second point to be measured. As the target moves, the reflected measurement light beam must continuously fall onto the photosensitive detector, continuously interfering with the reference beam. By electrically measuring the periodic variation of detector output, one can measure lengths as multiples and fractions of the wavelength of the illuminating source.
The reflective target must be continuously oriented to return the reflected beam to the active photosensitive target of the detector. Typically, the interferometry apparatus (source, photodetector module and measurement target) is fixed to an optical rail so as to allow free movement along an axis without requiring the operator to constantly align the reflect or towards the detector. The accuracy with which the reflector must be oriented will depend on the type of reflector used. One type of reflector is a flat, front surface mirror. The flat mirror gives the highest accuracy but has the most stringent alignment requirements.
A second type of reflector is a corner cube reflector. The cornercube reflector may have slightly less accuracy but requires only that the operator align the device so as to point towards the detector within a cone of perhaps 30-40 degrees in width. The reflected beam for the cornercube will emerge traveling parallel to the incoming beam, in the opposite direction and displaced from the incoming beam by a distance of twice the distance from the incoming beam to the cube center. If the apparatus is not in fixed alignment due to the presence of an optical rail, then it is the responsibility of the operator to continuously maintain alignment of reflector towards the detector.
A second type of interferometry apparatus, a tracking laser interferometer, contains mechanisms that allow the user to measure the distance to a target that is not fixed to a sliding rail. This device can steer the beam in azimuth and elevation so as to constantly track the distant target. However, the reflective target must still be manually oriented during movements of the target so as to keep the reflected measurement beam aligned towards the tracking interferometer detector. The device contains circuitry to readout the distance as well as the azimuth and elevation angles, thus enabling three dimensional coordinates to be directly read out.
Accordingly, there is a need for a device which accurately and continuously measures distances between two points in such a way as to lessen the need for continuous reflective target orientation adjustment by an operator.